The above-referenced application Ser. No. 686,203 describes and claims interface apparatus for enabling conventional automotive diagnostic and test equipment to be used with certain types of direct ignition systems, also referred to as wasted spark or distributorless ignition systems. In a wasted spark ignition system an individual ignition coil, each consisting of a primary winding and a secondary winding, is provided for each pair of cylinders, with the ends of the secondary windings being connected to the spark plugs of the respective cylinders. A six cylinder engine, therefore, has six spark plugs supplied by three secondary coil windings and there is no ground voltage reference for the secondary coil windings. When an individual ignition coil secondary winding is "fired", opposite polarity voltages are developed across its ends, and the two spark plugs connected to the secondary winding are fired together. The cylinder pairs are arranged so that one spark plug fires near the end of the compression stroke of its cylinder and its opposite paired spark plug fires near the end of the exhaust stroke of its cylinder. In practice, the wasted spark, that is, the spark produced near the end of the exhaust stroke of its cylinder, has a smaller amplitude than its counterpart power spark because it is not exposed to a gasoline/air mixture that is under compression. The wasted spark actually occurs earlier since the breakdown voltage of the exhaust mixture is significantly lower than the breakdown strength of the charged (pressurized) gasoline/air mixture. Since both spark plugs are connected in series through the ignition coil secondary winding, the wasted spark circuit is initially completed through the parasitic capacitances of the system, that is, the capacitances of the spark plug, spark plug wires, etc. to ground. Upon subsequent occurrence of the power spark, its conductive path includes the parasitic capacitances in addition to the discharge path through the other spark plug.
Conventional engine analyzers generally employ a secondary ignition system pattern pickup lead to obtain information regarding the performance of the secondary ignition system of the engine for processing and analysis. This information is typically sensed using a capacitive clamp-on pickup around the secondary coil wire between the ignition coil and the distributor. On an ignition system employing an integrated coil and distributor, such as the General Motors Corporation HEI system, use of a capacitive plate adapter may be required.
In a conventional engine, there is only one cylinder firing event per cylinder for each complete engine cycle (two engine revolutions in a four-cycle engine) and hence only one cylinder firing at a time. Also, all cylinder firings are in the same polarity direction. Consequently, there is no confusion as to what secondary signals are displayed when the secondary pattern pickup lead that is connected to a tester develops a signal. In a wasted spark system, the secondary windings of the ignition coils are each floating, with respect to engine ground, between a pair of spark plugs. This is in contrast to the autotransformer type of ignition coil in a conventional engine, which has a fixed engine ground reference. The result is that the voltages appearing on the secondary coils in a wasted spark system fluctuate with respect to engine ground. This could lead to misinterpretation of signals displayed on the screen and possibly erroneous data being processed by the engine analyzer. In the wasted spark system, two simultaneous firing events occur, one for the power firing of each cylinder and another for the wasted firing which occurs during the power firing of its complementary pair cylinder.
In both of the disclosed apparatuses of the above-referenced applications, the cylinder #1 power firing event signal is differentiated from the cylinder #1 wasted firing event signal so that proper synchronizing information is applied to the tester. A power firing event is herein defined as that corresponding to the cylinder being spark ignited near the end of its compression stroke (or near the beginning of its power stroke) and a wasted firing event is that which occurs when the cylinder is fired near the end of its exhaust stroke (or near the beginning of its intake stroke). Since, in a normally operating engine, the power firing event signals are larger than the wasted firing event signals, they can generally be differentiated based upon amplitude. In U.S. Pat. No. 4,644,284, the signals are compared with a fixed reference to determine which are the power firing event signals, whereas in application Ser. No. 941,630, the different firing event signals from all cylinders are sorted into two separate groups and a weighted average of the relative amplitudes of the two groups is obtained to determine the group that represents the power firing events and the group that represents the wasted firing events. In this way, one or two abnormally operating cylinders will not significantly affect the outcome of the determination.
The signal amplitudes are, of course, dependent upon spark plug condition and the type of vehicle. Also, since the wasted spark path and the power spark path are substantially coextensive, a problem in either of the two cylinders, in either of the two spark plugs or in the wiring will have a decided effect on the signal waveforms captured.
In the past, the secondary power firing waveforms have been processed to capture secondary ignition information as well as being displayed on the cathode ray tube (CRT) of a tester. The ignition data and their waveforms have provided valuable assistance in diagnosing engine problems. The difficulty is that in wasted spark ignition systems, the interrelationship between the power spark and the wasted spark significantly and adversely affects the signals and information captured, both for display and processing. It is therefore highly desirable to have some means for compensating for this interrelationship and to provide stable, accurate waveforms for processing and display of the wasted firing event signals and the power firing event signal.